The primary purpose of an alternating current (AC) motor drive is to provide a requested torque to the motor shaft. Ideally, the provided torque is constant with no distortion or ripple. Toward this end, the typical motor drive attempts to provide a balanced set of purely sinusoidal currents to the motor stator windings. However, due to practical design constraints of the AC motor, torque ripple will exist even with purely sinusoidal stator current excitation. Motor designers usually attempt to minimize the torque ripple generated by the AC motor. This can be accomplished by paying particular attention to design aspects such as winding configuration, stator tooth geometry, rotor barrier geometry, and rotor skewing. However, there exists a trade-off between torque ripple and torque density of the AC motor. Hence, in all practical applications the AC motor will produce some torque ripple when supplied by a sinusoidal current.
Depending upon the application, torque ripple can have certain adverse affects. For example, the torque ripple can cause speed ripple or excite driveline resonances. In the case of an electric or hybrid vehicle, this can result in vehicle oscillations which are a disturbance to the occupants. Active damping algorithms are often employed to counter these adverse affects. Additionally, stator vibrations and acoustic noise can be generated by the radial forces imposed on the stator laminations. Mitigating acoustic noise by passive means, such as the addition of structural reinforcement or sound dampening materials, can be a costly and undesirable solution. For these reasons, it is desired to develop a software based solution to minimize torque ripple, stator vibration, and acoustic noise.
Accordingly, it is desirable to provide a method and apparatus to reduce the stator radial forces in order to decrease vibration and acoustic noise. In addition, it is desirable to cancel or reduce a selected torque ripple harmonic. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.